The temperature of the gases released by the combustion process in a gas turbine engine may peak above 2100° C. and average 1500° C. This is much higher than the melting point of combustion chamber materials. Thus many gas turbine engine combustors employ ceramic or ceramic-coated tiles to line the combustor wall.
The individual tiles are attached to the combustor wall (e.g. the annular “cold skin” which forms the sides of the combustor) and cooling air passes through holes in the wall to impinge on the cold side of the tiles. The air then moves through a series of pedestals projecting from the cold side of the tiles to improve the convective heat transfer coefficient.
FIG. 1 shows a cut-away section through a combustor and illustrates a conventional fastening arrangement for a lining tile. The tile 1 has a threaded fixing stud 3, which extends from the cold side of the tile body 5 and passes through a hole 7 in the combustor wall 9. A washer 11 is placed over the stud and a nut 13 screws onto the stud to pull the tile towards the combustor wall, pedestals 15 on the cold side of the tile body determining the final spacing between the tile body and the wall.
Alternative tile fixing arrangements are proposed in U.S. Pat. No. 4,085,580 and U.S. Pat. No. 5,079,915.
To produce higher engine efficiencies and reduce emissions, there is a tendency for engine cycles to increase in temperature. The higher combustor temperatures, which this requires can require different cooling arrangements. In particular, pedestal cooling may be superseded by impingement effusion tile cooling arrangements. In such arrangements, the tile body has an array of effusion cooling through-holes, at a low angle (e.g. about 20°) to the surface. Cooling air effusing from the holes forms a protective cooling layer on the hot side of the tiles.
The cooling air effusion holes may be formed by laser machining of the tile body. However, this presents a problem in that laser machining is a line of sight process. As illustrated in the diagram of FIG. 2, which is a schematic cross-section through a tile, the fixing stud 3 obstructs laser drilling of effusion holes 17 in an area of the tile body 5 around the stud because of interference of the stud with the laser tool and/or the path (dashed, single headed arrow lines) of the laser beam. Thus the result can be a significant area (the extent of which is indicated by the double headed arrow line) around each stud that is devoid of cooling holes. Alternatively, approach vectors may have to be defined for the laser that avoid interference with the studs to produce holes in the vicinity of the studs. This requires extra programming and extra machining time, and may produce sub-optimal X, Y positions of the holes on the surface of the tile and/or sub-optimal angles of the holes to the surface.